This invention relates to stretch wrap thermoplastic film, methods for forming a unitized plurality of goods, e.g., a pallet load, by application of stretch wrap film thereto and the stretch-wrapped units resulting therefrom. The invention is particularly concerned with a stretch wrap film based on linear low density polyethylene (LLDPE) possessing a significant cling force due to the presence of a relatively high level of n-hexane extractibles therein. The invention is also concerned with a stretch wrap film having an exceptionally low unwind noise level.
The use of thermoplastic stretch wrap film for the overwrap packaging of goods and, in particular, the unitizing of pallet loads, constitutes a commercially significant application of polymer film. Overwrapping a plurality of articles to provide a unitized load can be achieved by a variety of techniques. In one procedure, the load to be wrapped is positioned upon a platform, or turntable, which is made to rotate and in so doing, to take up stretch wrap film supplied from a continuous roll. Braking tension is applied to the film roll so that the film is continuously subjected to a stretching, or tensioning, force as it wraps around the rotating load in overlapping layers. Generally, the stretch wrap film is supplied from a vertically arranged roll positioned adjacent to the rotating pallet load. Rotational speeds of from about 5 to about 50 revolutions per minute are common. At the completion of the overwrap operation, the turntable is completely stopped and the film is cut and attached to an underlying layer of film employing tack sealing, adhesive tape, spray adhesives, etc. Depending upon the width of the stretch wrap roll, the load being overlapped can be shrouded in the film while the vertically arranged film roll remains in a fixed position. Alternatively, the film roll, for example, in the case of relatively narrow film widths and relatively wide pallet loads, can be made to move in a vertical direction as the load is being overwrapped whereby a spiral wrapping effect is achieved on the packaged goods.
Some resins which have been used in the fabrication of stretch wrap film are polyethylene, polyvinyl chloride and ethylene vinyl acetate. A fairly recent development has been the utilization of linear low density polyethylene (LLDPE) in the manufacture of stretch wrap film, e.g., as described in U.S. Pat. Nos. 4,399,180, 4,418,114 and 4,518,654, the contents of which are incorporated by reference herein. The excellent toughness and puncture resistance properties of LLDPE makes it an excellent resin for such an application.
LLDPE and methods for its manufacture are described in, among others, U.S. Pat. Nos. 3,645,992; 4,076,698; 4,011,382; 4,163,831; 4,205,021; 4,302,565; 4,302,566; 4,359,561; 4,521,573 and 4,522,987. Although it is known from these disclosures that LLDPE can be extruded into films, there is no mention or suggestion in any of them that a useful stretch wrap possessing an inherent cling property can be manufactured from an LLDPE having a relatively high level of n-hexane extractibles, e.g., an amount of such extractibles of at least 3.5 weight percent for a film of about 1.5 mils thickness. The LLDPE films of commerce typically possesses levels of n-hexane extractibles which are well below this.
Thermoplastic films possessing a cling property are known in the art. U.S. Pat. No. 4,311,808 describes a cling film containing a homogeneous mixture of polyisobutylene, ethylene-propylene and a low density polyethylene. There is no specific mention of an LLDPE resin component in the film of this patent. U.S. Pat. No. 4,367,256 describes a cling wrap plastic film based on a blend of high pressure low density polyethylene (HPLDPE) and LLDPE in which the latter resin represents from 5-16 weight percent of the total. In one embodiment, this film is sandwiched between two HPLDPE films. Although a 100% LLDPE film is disclosed for comparison purposes, no mention is made of its n-hexane extractibles content. U.S. Pat. No. 4,399,173 describes a multilayer plastic film free of melt fracture which is suitable for a variety of applications including, by implication, the stretch wrapping of goods. The film possesses a core layer of LLDPE resin of melt index 0.2-3.0 decigrams per minute and skin layers of LLDPE resin of melt index 5.0-20.0 decigrams per minute. The patent lacks any disclosure or suggestion of an LLDPE film possessing a cling property as a result of the presence of a relatively high level of n-hexane extractibles. U.S. Pat. Nos. 4,399,180 and 4,418,114 describe a coextruded composite stretch wrap film in which an LLDPE core layer is surfaced with HPLDPE skin layers. In the one-sided cling stretch wrap film of U.S. Pat. No. 4,518,654, layer A fabricated from a resin possessing an inherent cling property and/or a cling property resulting from the incorporation of a cling additive therein is coextruded with layer B fabricated from a resin exhibiting little if any cling. In each of the two working examples of this patent, layer A is an LLDPE film containing a cling additive, i.e., polyisobutylene in both cases. There is no hint whatever in this patent of an LLDPE film possessing an inherent cling property resulting from the presence of a relatively high level of n-hexane extractibles. U.S. Pat. No. 4,436,788 describes a stretch wrap film obtained from a mixture of 40-90 weight percent ethylene-vinyl acetate copolymer and 8-55 weight percent LLDPE. No mention is made of a relatively high n-hexane extractibles LLDPE component. Japanese Laid-Open Patent No. 19528/1980 describes a two-layer stretch wrap film in which a low density polyethylene layer is coextruded with a low density polyethylene layer containing from 3 to 20 weight parts of a tackifier, e.g., a polyisobutylene having a molecular weight of 200 to 300. The use of an LLDPE resin containing a relatively high level of n-hexane extractibles is neither mentioned nor suggested in this disclosure.
As previously indicated, methods of stretch wrapping articles, containers, etc., are known. U.S. Pat. No. 3,986,611 describes a tension-wrapped palletized load obtained with a stretch wrap film possessing a cling additive. LLDPE films are neither disclosed nor suggested in this patent. U.S. Pat. No. 4,079,565 describes a stretch-wrapped package, process and apparatus which employs a stretch wrap polyethylene film. No mention whatever is made of an LLDPE stretch wrap film much less one possessing an inherent cling property. U.S. Pat. No. 4,409,776 discloses a method and apparatus for packing articles with a composite stretch wrap film one surface of which is nonadhesive. The adhesive surface is obtained with an "adhesive film" such as one fabricated from ethylene-vinyl acetate copolymer, 1,2-polybutadiene or styrenebutadiene copolymer and the nonadhesive surface is obtained with a "nonadhesive film" such as one fabricated from a crystalline polyolefin, e.g., polyethylene, polypropylene or ethylene-propylene copolymer. There is nothing in this patent which would suggest the use of an LLDPE possessing an inherent cling property (or "adhesive" characteristic to use patentee's term). If anything, U.S. Pat. No. 4,409,776 suggests that an LLDPE resin should be reserved for the "nonadhesive", which is to say, non-cling, layer of the composite film.
It is an object of the present invention to provide a monolayer LLDPE stretch wrap film possessing a significant inherent cling property on at least one of its major surfaces without the addition of a cling additive to the film.
It is another object of the present invention to provide a coextruded multilayer stretch wrap film in which at least one outer film layer is fabricated from an LLDPE imparting an inherent cling property to the exposed surface of the layer in the absence of cling additive.
It is still a further object of this invention to provide a coextruded AB stretch wrap film structure in which film layer A is derived from an LLDPE inherently exhibiting an appreciable level of cling without the use of cling additive and film layer B is derived from a resin which exhibits no appreciable cling to a layer of itself and optionally, has a slide property when in contact with a layer of itself with relative motion therebetween.
It is yet another object of the invention to provide a stretch wrap cling film having the lowest known unwind noise level.
Other objects of the invention include the use of a stretch wrap film of the aforementioned characteristics in the overwrapping of a plurality of goods, e.g., a pallet load, to provide a unitized packaged unit.